Secure LDAP uses LDAPS over TCP port 636 to encrypt directory communication between domain services and client systems.

IPsec (Internet Protocol Security) is the most secure way to provide site-to-site connectivity. It provides robust security services, such as data integrity, authentication, and encryption, ensuring that data sent across the network is protected from interception and tampering. Unlike other options, IPsec operates at the network layer and can secure all traffic that crosses the IP network, making it the most comprehensive and secure choice for site-to-site VPNs.References: CompTIA Network+ study materials and NIST Special Publication
800-77.

The requirement is to support over 300 hosts. The subnet mask 255.255.254.0 (or /23) provides 512 addresses, 510 of which are usable - ideal for around 300 devices.
* 255.255.0.0 (/16) provides too many addresses.
* 255.255.192.0 (/18) gives 16384 addresses - overkill.
* 255.255.255.254 is invalid for host assignments (only 2 addresses, 0 usable).
From Andrew Ramdayal's guide:
"To support 300 hosts, a /23 subnet (255.255.254.0) offers 510 usable addresses - the most efficient choice without excessive overhead."

Route selection between different sources is determined first by administrative distance. Ensuring the OSPF route has a lower administrative distance than the static route makes the OSPF- learned route preferred.